Controllers are frequently used in industrial and agricultural control applications to regulate and monitor parameters such as temperature, pressure, flow rate, level, and/or position. In HVAC systems, for example, such controllers are often used in conjunction with an air handling system to control the positioning of one or more actuator components for regulating the air flow distribution and/or air quality within a building or structure. Examples of such HVAC actuator components may include, but are not limited to, fan motors, dampers, compressors, air dryers, heaters, coolers, filters, humidifiers, and/or dehumidifiers.
The control of such actuator components is typically accomplished using a number of pneumatically and/or electrically actuated control valves, which can be actuated between an on-off position, between multiple discrete positions, or across an infinite range of positions. In the control of water temperature in a heat exchanger, for example, a two-position or modulating control valve such as a butterfly valve may be utilized to regulate the flow and/or temperature of hot water delivered through the exchanger. Other actuator components such as globe valves, zone valves, ball valves, solenoid valves, etc. may be similarly provided to control other aspects of the air handling system, as needed. In other applications, such actuator components may include devices such as temperature sensors, pressure sensors, flow rate sensors, liquid level sensors, proximity sensors, etc., which can be utilized in some industrial sensing applications to sense parameters within a duct, pipe, building or other desired location.
To provide a modulating output for those actuator components configured to accept a modulated control signal, a modulating controller is typically used to convert a duty cycle signal produced by the controller's microprocessor into an modulated analog output signal that can be used to directly interface with one or more modulated actuator components. In the control of modulated dampers and valves used in some HVAC systems, for example, a modulating controller or controller sub-base may be used to provide a modulated controller output of between 4 to 20 milliamps (mA) and 2 to 10 volts. While these control parameters are popular ranges for many modulated dampers and valves used in HVAC systems, other ranges and/or output types other than the standard 4-20 mA controller output can be provided.
To generate an analog modulated output signal (e.g. 4-20 mA), many controllers utilize microprocessors that are configured to output a pulse-width modulated (PWM) signal that is then fed to a digital to analog (D/A) conversion circuit. Special analog driver circuits may then be utilized to provide the desired modulated output (e.g. 4-20 mA) to each modulated actuator component. The use of D/A converters and special analog circuitry to provide the modulating output is often expensive, adding to the complexity of the microprocessor and associated drive electronics. In some cases, the heat generated by such electrical components can also affect the performance of the device, requiring the use of additional heat sinks and/or control circuitry to reduce heat. As such, many controllers must first be calibrated at the factory and/or on-site to ensure proper operation with the other system components.